Conventionally, a reduction of the weight of an automotive aluminum heat exchanger has been demanded in order to improve the fuel efficiency of the engine. In order to deal with this demand, it is necessary to reduce the thickness of the constituent material for the heat exchanger, such as a tube. However, a reduction of the thickness of the material tends to cause the leakage of fluid (refrigerant) due to pitting corrosion of the aluminum material. Therefore, it is important to reduce the thickness of the material while securing its corrosion resistance.
In a conventional automotive heat exchanger, the corrosion resistance of the tube which makes up a refrigerant passage is improved by extruding an aluminum material to have a multi-cavity flat cross-sectional shape, and forming a zinc diffusion layer having a surface zinc concentration of 0.5% or more on the outer surface of the multi-cavity flat tube by spraying zinc. The basic principle of this corrosion prevention method is as follows. Specifically, since the natural potential of the zinc diffusion layer is less noble than that of the aluminum material which makes up the tube, the zinc diffusion layer preferentially (sacrificially) corrodes in comparison with the aluminum material, thereby effectively preventing the pitting corrosion of the tube made of aluminum.
However, in this corrosion prevention method, since zinc is caused to adhere to the outer surface of the tube by spraying, it is difficult to cause the zinc to uniformly adhere to the surface of the aluminum material. If the amount of zinc caused to adhere by spraying is reduced from the economical point of view, zinc may not be supplied to part of the surface of the aluminum material, whereby the occurrence of corrosion concentrates on the part in which zinc is not supplied.
If the amount of zinc caused to adhere to the surface of the aluminum material is increased in order to prevent the occurrence of the above problem without taking cost into consideration, a high concentration of zinc is present in a brazed section between the tube and a corrugated fin. This may cause the brazed section to selectively corrode, whereby the corrugated fin may be removed from the tube.
The applicant of the present invention has disclosed a brazing method in which a conventional anticorrosive flux such as potassium fluoroaluminate is applied to a tube by using an organic binder in which an oxirane group-containing resin and polyoxazoline are added to a methacrylate copolymer (Japanese Patent Application Laid-open No. 2000-687).
The applicant of the present invention has also disclosed a method of applying a mixture of an organic binder containing a methacrylate polymer or copolymer as a major component, an anticorrosive flux, and silicon or zinc powder to an aluminum-extruded multi-cavity flat tube for an automotive heat exchanger by using a roll transfer method (Japanese Patent Application Laid-open No. 11-239867).
The present inventors expected that zinc could be uniformly and economically supplied to the surface of the aluminum material by applying the above conventional technology to a tube for a heat exchanger, and applying zinc powder to the tube through an organic binder. The present inventors prepared a paint (brazing composition) by mixing an organic binder, an anticorrosive flux, and zinc powder by way of experiment, and attempted to apply the paint to the tube. However, since the viscosity of the paint was increased within several hours after preparation, the paint could not be stably applied to the tube.
This phenomenon also occurs in a paint which does not contain an anticorrosive flux. Therefore, it is estimated that the viscosity of the paint is increased due to a chemical reaction between the organic binder and the zinc powder.
In more detail, it is estimated that a complexation reaction occurs between a carboxyl group in the organic binder and a zinc ion formed by hydrolysis of the zinc powder, and the organic binder forms a three-dimensional structure having the zinc ion as a nucleus, whereby the viscosity of the paint is increased. The carboxyl group in the organic binder is a substituent necessary for making the binder water-soluble taking recent environmental protection into consideration, and cannot be removed from the binder.
An increase in viscosity may be prevented by inhibiting formation of the zinc ion by using a zinc fluoride, which is a metal compound more stable than the zinc powder (powder of element zinc), instead of the zinc powder. Published Japanese Translation of PCT International Publication for Patent Application No. 2002-507488 discloses an alkaline metal salt of a fluorozincate, such as KZnF3, as a specific example of the zinc fluoride. A fluoride has a low corrosiveness to a metal and has an effect as the flux.
The present inventors prepared a paint by mixing the above aqueous organic binder, potassium fluoroaluminate (flux), and KZnF3. As a result, the increase in viscosity of the paint was reduced in comparison with the case of using zinc powder. However, the viscosity of the paint was considerably increased within about one day after preparation. It is estimated that the increase in viscosity is caused by the formation of Zn2+ due to hydrolysis of part of the KZnF3. Therefore, although the paint can be applied immediately after its preparation, there still remains a great degree of limitation from the industrial point of view.
International Patent Publication No. WO01/38040A1 discloses an aluminum brazing composition which includes KZnF3 (zinc fluoride), a methacrylate homopolymer or copolymer, and an organic solvent such as xylene, and a brazing method using the same. This application suggests the use of a methacrylate homopolymer which does not contain a carboxyl group as a binder, for example. However, since the glass transition temperature of a homopolymer tends to be increased as the number of carbon atoms is decreased, the brazing composition may be removed from the aluminum material or cracks may occur on the surface of the brazing composition. Moreover, the use of toxic organic solvents has been limited from the viewpoint of environmental protection and safety in recent years. In particular, the use of aromatic solvents such as xylene gives rise to considerable problems.
The present inventors have conducted extensive studies to solve the problems relating to the interaction between the zinc ion and the carboxyl group in the organic binder based on the above knowledge. As a result, the present inventors have proposed an aluminum brazing method capable of controlling the reactivity of the carboxyl group in the organic binder by adding a specific compound such as a tertiary amino ethanol, such as dimethylamino ethanol or diethylamino ethanol, or a secondary amino alcohol, such as methylamino ethanol or ethylamino ethanol, as a reaction inhibitor (Japanese Patent Application No. 2001-193372).
Since the nitrogen atom in the amino alcohol has an unpaired electron, the amino alcohol basically functions to preferentially react with a proton-dissociated carboxyl group in the organic binder, whereby a reaction between the zinc ion formed by hydrolysis and the organic binder can be inhibited.
Therefore, an aluminum brazing composition containing an amino alcohol together with the zinc-based flux (KZnF3) improves the corrosion resistance by a sacrificial corrosion effect of a zinc diffusion layer, and inhibits a reaction between the organic binder and the zinc ion by the effect of the amino alcohol, whereby an increase in viscosity of the brazing composition can be prevented.
However, since the specific gravity of KZnF3 in the brazing composition is extremely large, KZnF3 precipitates within several hours after preparation of a paint. This not only makes it necessary to stir the paint at the time of application, but also causes the paint to be solidified at the bottom of the container through the binder during storage. This precipitation phenomenon also occurs in a relatively short period of time until the paint is applied to the aluminum material. Therefore, a difference in concentration is produced in the paint, whereby the paint cannot be uniformly applied.